First, the system I have done this on: Late 2013 27" iMac 3.2 GHz quad core i5 32 GB RAM Toshiba 256 GB SSD boot drive on Thunderbolt Port Thunderbolt Display OSX 10.10.4 Parallels 10.2.2 I have done the following steps: 1. From Parallels, created a clone of my Windows 8.1 Pro virtual machine. Virtual Machine Config: RAM -> 8GB Expanding Virtual Disk -> 100 GB (43 GB used) Video Memory -> 512 MB Cores -> 2 Network adapter -> Intel PRO/1000MT 2. From the Windows 10 download site (https://www.microsoft.com/en-us/software-download/windows10ISO) I downloaded the tool from inside the Windows 8.1 clone. 3. Started the tool and downloaded the 64 bit Windows 10 Pro .iso file 4. When that completed, I copied the .iso file to a folder on the OS X side (for convenience I put it in the default Parallels folder) and then removed it from the VM. 5. Just to make sure all was starting fresh, I rebooted the Win8.1 Pro VM. 6. After the reboot and log in, from the Parallels device menu I chose Devices->CD/DVD->Connect Image... 7. Navigated to and selected the .iso file. 8. In Windows I was prompted with the "What to do about this" dialog box when the .iso file mounted and selected run Windows setup (or whatever it was called). 9. Then proceeded through some of the file copies and when got to a point as to what to do about a particular file, clicked the "Change what to keep" link. 10. From the choices I selected keep nothing and hit continue. 11. After the usual copy files, reboot install files reboot, etc. cycles I finally got to the windows set up stuff and selected express set up. 12. Once this completed and got to the desktop, the display was what looked like a default 1024/768 or maybe even 640/480, but was not alarmed as a clean installed wiped all the Parallels tools (probably). Waited first for all my One Drive files to sync and then reinstalled Parallels tools, rebooted. 13. Success, rebooted to full resolution of my Thunderbolt display and at least the new Windows web browser is working (indicates network is functioning) and access to my One Drive. So now, time to start installing my apps and see what happens!